纳米晶钛酸钡陶瓷的制备、微结构及性能的研究

论文题目：纳米晶钛酸钡陶瓷的制备、微结构及性能的研究作者简介：邓湘云，女，1964年11月出生，2003年9月师从于清华大学李龙土教授，于2007年1月获博士学位。

中文摘要自从1943年钛酸钡作为最具有代表性的钙钛矿结构的铁电材料被发现以来，一直是电子陶瓷元器件的基础材料。

当高于居里温度120o C时，钛酸钡晶体结构为立方顺电相; 低于居里温度时钛酸钡有三个结构相变点; 在大约10o C到130o C之间为四方相结构; 低于10o C为正交相结构, 当温度进一步下降到大约-80o C表现为三方相结构。

近几年来随着电子及微电子工业的飞速发展，多层陶瓷电容器的微型化和大容量化要求降低陶瓷介质层的厚度，这就要求介质层中的陶瓷晶粒降到亚微米级甚至纳米级, 因此制备小粒径的钛酸钡陶瓷引起了人们广泛的兴趣。

然而高致密的陶瓷都要通过高温烧结才能致密化，而致密化过程和晶粒生长过程常常同时产生，特别在烧结后期晶粒生长非常迅速，其结果是材料实现致密化后晶粒也长大了。

因此目前最大的研究障碍就是制备出致密的纳米晶陶瓷，并在此基础上研究晶粒尺寸对钛酸钡陶瓷的微结构和性能的影响,即纳米尺寸效应。

钛酸钡陶瓷尺寸效应研究始于1950年；研究的核心内容主要围绕晶粒尺寸对于介电性能，相变和显微结构的影响。

研究表明随着晶粒尺寸的减小，居里温度向低温移动; 晶粒尺寸从10µm减至1µm时，室温介电常数增大，当钛酸钡陶瓷的平均晶粒尺寸接近1µm 时，介电常数特别大; 当陶瓷的晶粒尺寸小于500nm之后，相对介电常数迅速下降。

Zhao Zhe等研究表明50nm钛酸钡陶瓷1kHz时室温介电常数为1100，四方相→立方相的温度为117o C; Buscaglia等采用拉曼光谱在80-700K温度区间内对50nm钛酸钡陶瓷的相结构研究，证实了在50nm钛酸钡陶瓷中依然存在和大晶粒钛酸钡陶瓷相同的相变行为，即随着温度的降低，经历由立方→四方→正交→三方的相转变，并且存在多相共存的特点; 并且证实30nm钛酸钡陶瓷100Hz时70o C介电常数为1650，四方相→立方相的温度为106o C; 他们还研究了30nm钛酸钡陶瓷中铁电畴的分布，并观察到一畴多粒现象。

尺寸效应研究中有关钛酸钡陶瓷保持铁电性临界尺寸的内容尤其受到关注, 因为当晶粒尺寸减小到临界尺寸时，铁电性将消失。

1954年首次报导了铁电性消失的现象;1985年Arl证实钛酸钡陶瓷保持铁电性的临界尺寸为280nm; M. H. Frey等用热压（压力为8GPa）的方法制备出70nm相对密度98％的钛酸钡陶瓷，并认为临界尺寸为70nm; Zhao zhe等采用SPS制备出50nm相对密度97％的钛酸钡陶瓷，并实验证明50nm钛酸钡陶瓷依然保持铁电性；2006年M. T. Buscaglia等报导了SPS方法制备的钛酸钡陶瓷，平均晶粒为30nm，相对密度为97％，并认为钛酸钡陶瓷维持铁电性的最小尺寸为30nm。

由于制备技术的限制，难于获得高密度的钛酸钡陶瓷，所以钛酸钡陶瓷铁电性维持的临界尺寸至今未得到实验验证。

Zhao Zhe等基于朗道理论预测钛酸钡陶瓷铁电性消失的临界尺寸为10-30nm。

本研究利用两段式无压烧结和放电等离子体烧结工艺分别制备出一系列晶粒尺寸小于100nm的高致密度纳米晶钛酸钡陶瓷，其中采用两段式无压烧结制备出的8nm 钛酸钡纳米晶陶瓷，其相对密度高达99.6%, 是目前世界上报导的最小尺寸的致密铁电陶瓷块体材料; 通过放电等离子体烧结工艺制备出的钛酸钡纳米晶陶瓷最小晶粒尺寸为15nm, 目前国际上用同样方法制备的钛酸钡陶瓷最小尺寸是30nm。

论文系统研究了纳米晶钛酸钡陶瓷的微结构和物理性能，在实验基础上对纳米晶钛酸钡陶瓷的尺寸效应进行了深入分析与研究, 发现纳米晶钛酸钡陶瓷在晶粒尺寸小到8nm时仍具有铁电性，突破了人们认识的钛酸钡陶瓷铁电性最小临界尺寸30nm 的现状。

这对于钛酸钡纳米晶陶瓷材料在新型微电子器件中的应用具有重要的理论和实际意义，同时也对纳米晶铁电陶瓷理论体系的建立和完善提供了科学依据。

论文对于不同晶粒尺寸变温拉曼光谱的研究表明: 大晶粒的钛酸钡陶瓷中,在-150o C 存在清楚的三方相的特征软膜185 and 171cm-1, 171cm-1软膜在-100o C左右渐渐变弱并发生三方→正交相变, 485cm-1软膜在0o C 附近消失表明发生了正交→四方相变; 随着温度升高, 310和716cm-1软膜的强度在100o C附近开始下降, 大约在150o C 时消失, 表明四方→立方相变点为150o C。

而纳米晶钛酸钡陶瓷具有和大晶粒钛酸钡陶瓷相同的相变过程, 即随温度降低经历从立方→四方→正交→三方相变, 但是三方相不能从正交相中分辨出来,表示具有正交和三方的弥散型相变特征。

随着温度的升高, 相变的温度区间变得更加宽; 而且随着晶粒尺寸的减小, 多相共存的特征更加明显, 从-190o C到200o C的温度区间184cm-1软膜的存在, 说明正交相存在于很宽的温度区间, 305 and 715cm-1软膜在200o C依然存在表明纳米晶钛酸钡陶瓷在200o C 依然存在四方相。

进一步研究表明100nm以下的钛酸钡陶瓷均在200o C仍然保有少量的四方相，但纳米晶钛酸钡陶瓷相变区间弥散，呈现多相共存的结构特性；即使晶粒小到8nm的钛酸钡陶瓷，仍然存在与大晶粒钛酸钡陶瓷类似的相变行为即随温度的降低经历从立方→四方→正交→三方的相转变。

不同晶粒尺寸室温XRD的研究结果表明由于晶粒尺寸变化导致 (200)和(002)峰的劈裂。

可以看到当晶粒尺寸为80nm时，开始出现(200)和(002)峰劈裂，说明此时具有立方、四方相的共存结构。

当晶粒尺寸增加到3µm时，在2θ=45o附近的衍射峰位有明显的劈裂，这意味着钛酸钡的晶相结构随着晶粒尺寸的增加完全变成了四方相结构。

室温XRD数据表明50nm以下的纳米晶钛酸钡陶瓷的晶体结构具有准立方结构; 同步辐射原位超低温高分辨XRD研究结果表明50nm和80nm钛酸钡陶瓷存在多相共存, 弥散型结构特征。

采用透射电镜和场发射高分辩透射电镜观测到在100nm以下的钛酸钡陶瓷中有大量的纳米铁电畴。

利用扫描探针技术的非线性扫描介电显微(SNDM)发现100nm以下的钛酸钡陶瓷从25至290o C温度区间存在大量的纳米铁电畴，并通过原位变温观察到纳米铁电畴的变化趋势。

从20nm钛酸钡陶瓷不同温度的SNDM图和对应温度连线的剖面图中可以看出，SNDM图都表现为亮暗和灰色衬度，说明20nm钛酸钡陶瓷有复杂的电畴结构，从20nm钛酸钡陶瓷25到290o C不同温度电畴分布可以看出室温下20nm 钛酸钡陶瓷的铁电畴的大小约为4-32nm，随着温度从25o C升高到 100o C, 衬度区域变宽且衬度加强; 高于居里点从150o C开始, 大尺寸铁电畴的区域开始减少,小尺寸铁电畴区域增多, 从25到150o C温度区间有许多铁电畴的尺寸大于20nm, 说明20nm 钛酸钡陶瓷中有的晶粒中包含多个电畴；随着温度进一步升高, 尺寸大且亮的电畴区域不见了, 电畴尺寸的分布逐渐变窄; 当温度高于290o C时衬度区域非常明显,说明20nm钛酸钡陶瓷中依然存在铁电畴。

介电性能测试结果表明：随着晶粒尺寸减小，居里点向低温移动，另外两个相变点向高温移动; 8nm钛酸钡陶瓷室温仍具有较高的介电常数（1500）和低的介电损耗（0.02），而且相变区间非常宽，到290o C还依然保持铁电性; 晶粒尺寸小于100nm 时，钛酸钡陶瓷的介温谱呈现弥散型相变特征。

对100nm以下钛酸钡陶瓷从室温至290o C的不同温度的电滞回线的研究, 进一步证实了即使到290o C纳米晶钛酸钡陶瓷依然存在铁电性。

纳米压痕对硬度和蠕变的测试结果: 当晶粒尺寸高于50nm时，随着晶粒尺寸的减小，硬度和弹性模量增大; 当晶粒尺寸低于50nm时，晶粒尺寸减小硬度和弹性模量降低; 在钛酸钡陶瓷体系中晶粒尺寸不是影响钛酸钡陶瓷蠕变的唯一因素。

最后本研究从第一性原理分子动力学的角度对纳米晶钛酸钡陶瓷的铁电性起源和相变进行了初步探讨。

基于第一性原理对钛酸钡陶瓷电子结构的计算结果表明即使晶粒尺寸小到8nm，Ti-O之间仍然存在电子轨道的杂化，而这种杂化对于纳米晶钛酸钡陶瓷铁电性的保持有重要作用; 采用密度泛函理论广义梯度近似方法计算了不同晶粒尺寸钛酸钡陶瓷不同晶相的振动模式，结果表明8nm钛酸钡陶瓷和大晶粒钛酸钡陶瓷一样随温度的降低经历从立方→四方→正交→三方的相转变。

关键词：纳米晶钛酸钡陶瓷；制备；微结构；物理性能；尺寸效应Preparation, Microstructure and Properties of NanocrystallineBT CeramicsDeng XiangyunABSTRACTBaTiO3 (BT) has been the basic materials of electronic ceramic components, since it was found as the most representative perovskite structure ferroelectric material in 1943. Above the Curie temperature (T C=120°C), the structure of BT is cubic (C) and paraelectric. Below the Curie point, the structure is slightly distorted and three ferroelectric polymorphs with nonzero dipole moment exist depending on temperature. The tetragonal (T) modification is stable between about 10°C and 130°C. Below 10°C the structure becomes orthorhombic (O) and a further transition to rhombohedral (R) structure occurs around −80°C.With the rapid development of electronics and microelectronics industry in recent years, miniaturization and large capacity of multilayer ceramic capacitors require lowering ceramic medium layer thickness and submicron or nanometer grain in ceramic dielectric layer must be obtained, therefore small size BT ceramic preparation aroused widespread interest. However, the dense ceramic must be sintered at high-temperature. The densification and grain growth process often generated at the same time, grain growth is very rapid especially in the latter part of sintering, and the result is the realization of dense material after grain has grown up. The current biggest research obstacle is to prepare dense nanocrystalline ceramics, and then study the size effect on the micro-structure and properties.BT size effect study began in 1950; the core research content is about size effect on dielectric properties, phase transition and microstructure. The permittivity of BT ceramics increases with decreasing grain size, passing through a maximum around 1µm. On further decreasing the grain size below 500nm, a rapid decrease of relative permittivity is observed. The dielectric data show a broad ferro–para phase transition with a maximum permittivity of ≈1100 at 117o C and 1 kHz in 50nm BT ceramics. The obtained spectra in the range of 80–700K show the presence of all the crystalline phases in 50nm BT ceramics, and the phase transitions take place in a large range of temperatures in which the phases might coexist. Buscaglia et al. demonstrate permittivity of 1650 at 70o C at 100kH in 30nm BT ceramics. The existence of a ferroelectric domain structure with domains extending over several grains in 30nm BT ceramics given by piezoresponse forcemicroscopy.Among the size effect researches particular concern is the ferroelectricity critical size, because when the grain size reduces to the critical size, the ferroelectricity will disappear. The first report of the ferroelectricity disappearance was in 1954; Arlt revealed that BT critical size was 280nm in 1985; Frey et al. reported the dielectric data of dense BT ceramics with grain size of 70nm fabricated by pseudoisostatic hot pressing, and demonstrated the critical size was 70nm. Recently Zhao et al. and Buscaglia and coworkers obtained dense BT ceramics with grain sizes of 50 and 30nm, respectively, using spark plasma sintering, and reported the critical size was 50 and 30nm accordingly. Due to preparation technical constraints, it’s difficult to obtain high density BT ceramic, and the critical size has not been verified by experimental tests. The critical grain size corresponding to suppression of ferroelectricity is estimated to be 10–30nm in the framework of the Landau-Ginsburg-Devonshire theory.The bulk dense nanocrystalline BT ceramics below 100nm have been successfully prepared by the two step sintering and the spark plasma sintering process, respectively. Among them, 8nm nanocrystalline BT ceramics with the relative density of 99.6% was prepared by the two steps sintering for the first time, which is the minimum size dense ceramic bulk material in the world currently. The smallest grain size of 15nm nanocrystalline BT ceramics had also been successfully prepared by the spark plasma sintering method. At present the minimum size nanocrystalline BT ceramics is 30nm prepared by others using the spark plasma sintering method in the world.Microstructure and physical properties have been measured and the investigations on the size effect of BT ceramics have been comprehensively and systemically conducted. Ferroelectricity was being found in BT ceramics with grain size as small as 8nm in diameter, breaking the status that the BT ceramic critical size is 30nm, which has great theoretical and practical significance in the new microelectronic devices application of nanocrystalline BT ceramics, and also providing a scientific basis for the establishment and improvement of the nanocrystalline ferroelectric ceramics theoretical system.Temperature-dependent Raman spectra of different grain size BT ceramics showed that in coarse BT ceramics, the clear peak at 185 and 171cm-1 indicates the R phase at -150o C. The gradually weak mode at 171cm-1demonstrates the R→O transition near -100o C. 在The 485cm-1mode loses its intensity around the O→Ttransition at 0o C. As the temperature increases, the intensity of the bands at 310 and 716cm-1begins to decrease around 100o C and disappears near 150o C, which is attributed to the T→C phase transformation. The Raman spectra of nanograin BT ceramics are fundamentally the same C→T→O→R transitions as those in coarse BT ceramics, but the R phase cannot be distinguished from those recorded in the O phase, which manifests coexistence of both O and R phases and a diffuse feature. As the temperature increases, the temperature range of the phase transition becomes more broadening and the feature of multiphase coexistence is more obvious with decreasing grain size. The presence around the 184cm-1mode from -190to 200o C demonstrates that the O phase exists in a very wide temperature range. Note that the nominally “C” spectrum at 200o C still has weak features at 305 and 715cm-1, indicating T remnants.The similar successive transitions in 8nm nanograin BT ceramics from rhombohedral to orthorhombic, tetragonal and cubic transitions as those in coarse BT ceramics and the diffused phase transition, multiphases coexistence characters in nanocrystalline BT ceramics below 100nm.XRD patterns at room temperature of BT ceramics with different grain size showed that the grain size induced transition from global C→T symmetry demonstrated by the gradual splitting of pseudocubic into tetragonal (200) and (002) reflections. A slight long-range tetragonal lattice distortion first starts to appear in 80nm ceramics; the splitting of XRD peaks characteristic, which relates to the coexistence of cubic and tetragonal structure. For 3000nm BT ceramics, the (002)–(200) reflection clearly shows splitting peaks, normally exhibited by micrometric ceramics at room temperature. The diffraction patterns of nanocrystalline below 50nm BT ceramics apparently correspond to pseudo-Pm3m cubic symmetry revealed by XRD patterns at room temperature. Multiphase coexistence and the diffused phase transition character in 50 and 80nm nanocrystalline BT ceramics revealed by in-situ temperature dependent high resolution XRD.TEM and HRTEM observations also showed the presence of nano ferroelectric domains in BT ceramics below 100nm. Scanning nonlinear dielectric microscopy (SNDM) measurements confirmed the existence of nano ferroelectric domains from 25 to 290o C in BT ceramics below 100nm. It showed from the domain images of the 20nm BT ceramics by SNDM in different temperatures and the cross sectional images along the lines in the corresponding temperature. TheSNDM images reveal relatively strong black/white contrast, indicating the presence of a ferroelectric domain structure in the 20nm BT ceramics.It also revealed the domain width from 25 to 290°C from the cross sectional images along the lines in the corresponding temperature. It is found that the irregular spot domain patterns with the size from 4 to 32nm were clearly observed at 25°C. The black/white contrast regions became broadened and intensified with the temperature increasing from 25 to 100°C. From the beginning of 150°C above Curie temperature, the remarkable contrasted domain regions reduced and the small domain regions with opposite contrasts increased. It is also revealed that many domain sizes are bigger than 20nm from 25 to 150°C, demonstrating the existence of a ferroelectric domain extending over several grains for 20nm BT ceramics. With the temperature further increasing, the big and bright spot domain regions were invisible and the domain size distribution gradually turned to be narrow; however, the clear white and black contrast domain regions were obviously observed at 290°C, indicating the existence of nanoferroelectric domains above Curie temperature in 20nm BT ceramics.With decreasing grain size, the Curie temperature decrease to the low temperature; and the other two phase transition points shift to the high temperature. The dielectric test indicated that 8nm BT ceramics had a fairly large dielectric constant 1500 and small dissipation factor 0.02 at room temperature, with a broad phase transition maximum and ferroelectricity was retained at the high temperature of 290°C; the diffused phase transition characters in nanograin BT ceramics below 100nm were further approved. The piezoresponse hysteresis loops show clearly that below 100nm BT sample are still switchable and ferroelectricity retains at 25°C, and even at the high temperature of 290°C.The hardness of BT ceramics were determined by two trends: (a) hardness increased with the grain size decreasing at the grain size above 50nm, which was consistent with the Hall-Petch effect; (b) hardness decreased with the grain size decreasing at the grain size below 50nm, which was agreement with the inverse Hall-Petch effect.The calculation of BT ceramics electronic structures based on the first principle revealed the hybridization between Ti 3d and O 2p in 8nm BT ceramics, which was responsible for the tendency to ferroelectricity in nanograin BT ceramics.The vibrational modes of BT in different crystalline phases have beeninvestigated by means of density functional theory pseudo potential method. The evidences of T to O and O to R phase transitions present for 8nm BT ceramics, which further demonstrated the similar successive transitions in 8nm BT ceramics from R to O, T and C transitions as those in coarse BT ceramics.Key words: nanocrystalline BT ceramics; preparation; microstructure;property; size effect。